Collaborative Research: How to live on a (carbon and water) budget: Tree investment in chemical defenses across a gradient of physiological drought stress

干旱引起的森林死亡是一种全球现象，具有深远的生态和经济影响。在美国西部，干旱、高温和树皮甲虫爆发导致的树木死亡现在超过了森林的生长。尽管与干旱有关的昆虫爆发的频率和严重程度有所增加，但影响树木对食草昆虫易感性的因素，如化学防御的存在，却很少受到关注。特别是，目前还不清楚干旱是如何导致树木将资源转向或远离阻止昆虫攻击的化学防御，以及随后与其他重要植物功能存在的权衡。为了解决这一关键的知识缺口，本研究重点研究了在干旱胁迫加剧的情况下，pinon松树如何将其碳资源分配给防御和其他生理过程，以避免因干旱或树皮甲虫攻击而死亡。本研究在温室和田间实验中使用同位素标记方法来跟踪干旱引起的碳分配到影响树皮甲虫选择和成功的特定化合物的变化。该项目增加了美国土著学生的参与，通过招收本科生获得暑期研究助理奖学金，并通过开发两个批判性思维模块，支持在蒙大拿州农村中学的课堂上采用这些模块，增加了其他未被充分代表的少数民族在科学领域的参与。树木死亡率在全球范围内有所增加，但对树木死亡机制的理解仍然令人惊讶地有限。迄今为止，大多数与干旱相关的树木死亡研究都集中在理解碳饥饿和水力衰竭的耦合作用上，但干旱通常伴随着导致或促成树木死亡的昆虫爆发。研究还没有确定树木何时停止投资有效的化学防御来抵御干旱胁迫的生物攻击，以及这些碳可用性的变化如何同时影响植物的其他生理过程。本研究寻求对干旱胁迫如何影响树木水力功能、碳水化合物有效性和化学防御之间相互作用的机制理解。利用温室实验和田间干旱操作，本研究将结合酶分析和稳定同位素的使用，以确定对树皮甲虫行为有已知影响的单个化合物水平上pinon松树最近固定碳分配变化的机制。这些方法还可以识别在不同干旱严重程度下防御化合物合成的权衡，同时也可以促进对树木生理学和整棵树C预算的基本理解。通过对干旱生理胁迫对树皮甲虫防御机制的影响的全面了解，将建立一个新的、更完整的评估树木死亡机制的框架。本研究由IOS/BIO的综合生态生理学项目和促进竞争研究的既定项目（EPSCoR）共同资助。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Drought-induced forest die-off is a global phenomenon with far-reaching ecological and economic impacts. In the western US, tree death from drought, high temperatures, and bark beetle outbreaks now exceeds forest growth. Despite increases in the frequency and severity of drought-related insect outbreaks, factors influencing a tree's susceptibility to insect herbivores, such as the presence of chemical defenses, have received little attention. In particular, it is unclear how drought causes trees to shift resources toward or away from the chemical defenses that deter insect attacks and the subsequent trade-offs that exist with other important plant functions. To address this critical knowledge gap, this study focuses on how pinon pine trees allocate their carbon resources toward defense and other physiological processes under increasing drought stress to avoid death by drought or bark beetle attack. This study uses an isotope labeling approach in both greenhouse and field experiments to track drought-induced changes in carbon allocation to specific chemical compounds that affect bark beetle choice and success. The project increases participation of Native American students through undergraduate recruitment for summer research assistantships as well as other underrepresented minorities in science through the development of two critical thinking modules and support for their adoption in rural middle school classrooms across Montana. The rate of tree mortality has increased across the globe yet the understanding of the mechanisms underlying tree death remains surprisingly limited. Most work to date on drought-related tree death has focused on understanding the coupled roles of carbon starvation and hydraulic failure, but drought is oftentimes accompanied by insect outbreaks that cause or contribute to tree mortality. Research has yet to determine when trees cease investment in effective chemical defenses against biotic attack along the continuum of drought stress, and how these shifts in carbon availability simultaneously impact other plant physiological processes. This study seeks a mechanistic understanding of how drought stress affects the interactions among tree hydraulic function, carbohydrate availability, and chemical defense. Using both greenhouse experiments and field drought manipulations this research will couple enzyme assays with the use of stable isotopes to identify mechanisms responsible for shifts in pinon pine allocation of recently fixed carbon at the level of individual compounds with known impacts on bark beetle behavior. These methods will also allow identification of trade-offs involved in the synthesis of defense compounds at different drought severities while also advancing fundamental understanding of tree physiology and whole tree C budgets. By providing a comprehensive understanding of the effects of drought-induced physiological stress on mechanisms determining defense against bark beetles, a new, more complete framework for assessing mechanisms of tree mortality will be developed. This research was co-funded by the Integrated Ecological Physiology Program in IOS/BIO and by the Established Program to Stimulate Competitive Research (EPSCoR).This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.